8-14 PRECIPITATION HARDENING

8-14 PRECIPITATION HARDENING

As discussed previously (Chapter 3), the essential requirement for precipitation to occur in solid solution is the decreasing solubility of a solute with decreasing temperature. This results in a supersaturated solid solution that, being unstable, tends to decompose according to the relation

Supersaturated α Saturated α β precipitation solid solution

solid solution

(8.13) Figure 8-26a shows a phase diagram illustrating the type of alloy and conditions for

precipitation hardening.

o C 1 α + liquid

Solid solution

FIGURE 8-26 (a) Al—Cu partial equilibrium diagram. Only the part rich in aluminum is shown. (Adapted from Metals Handbook, 1948 edition. By permission of American Society for Metals.) (b) Precipitation hardening process showing schematically resulting microstructures. (1) Solid solution treatment: heat to a temperature of about 540°C (1004°F) (2) Quenching: cool rapidly in water at 20°C (78°F). (3) Age or precipitation hardening: reheat to a temperature of about 200°C (392°F) and hold 2 h. Fine precipitate results. (4)

Overaging results if temperature is too high and/or the time is too long. Fine particles coalesce to form coarse noncoherent precipitate causing a considerable decrease in strength.

In general, the second-phase β precipitate is formed by a sequence of thermal treatments. First, an alloy of suitable composition, as indicated by line xx in Figure 8-26, is subject to a solution heat treatment. This consists of heating the alloy to a temperature above the solvus line but below the solidus line in region α (point 1), and holding at this temperature for a sufficiently long time so that complete homogenization of the solid solution takes place. Then the alloy is quenched to about room temperature (point 2 on the diagram) so that precipitation of a solute in excess of equilibrium concentration does not occur and a supersaturated solution results. On quenching, the high concentration of vacancies corresponding to the equilibrium concentration at the solution temperature (point 1) is retained. These vacancies slowly precipitate or condense out, producing clusters and dislocation loops that form the sites at which precipitate particles can nucleate. Furthermore, the quenched-in vacancies enhance the diffusion rate of the solute, thereby promoting the formation of precipitate.

Quenching does not cause significant changes in mechanical properties, although some lattice strains may be present because of thermal stresses. The quenched alloy is relatively soft and may be worked to the desired shape without precipitation occurring. The third step, called age hardening or precipitation hardening, involves reheating the alloy to an elevated temperature somewhere below the solvus line (around point 3) and holding at this temperature, called aging temperature, for a certain period of time to develop the necessary amount and kind of precipitate that will impart a desired strength (see Fig. 8-26b).

The increase in the yield strength of the precipitation-hardenable alloy depends primarily on the amount of precipitate and its characteristics, such as the particles’ size, shape, and distribution. As the temperature of precipitation is lowered, the precipitate particles become smaller and more numerous and the strengthening effect is greater. With increasing temperature particle size increases and the precipitate continues to coalesce as aging progresses.

Precipitation hardening is a very important method of strengthening many solid solution alloys such as a variety of aluminum and magnesium alloys, copper—beryllium alloys, high-nickel-base alloys, and some stainless steels. The most effective way of strengthening such alloys is to use a solute element with a sharp change of solubility with temperature and in sufficient atomic concentration to produce large volumes of fine precipitate uniformly distributed within the grains. The aging temperature depends on the composition, structure, and type of alloy. At early stages of precipitation, the electrical resistivity of alloys increases but, at later stages of precipitation, the resistivity may decrease. The use of an age-hardenable alloy is restricted to temperatures in service, during which time overaging may not become excessive. It is apparent that the precipitation-hardenable alloy should never be used at temperatures above its solvus line since, at such temperatures, the precipitate will dissolve and complete homogenization of the solid solution will occur, completely eliminating the effect of hardening. To avoid these problems, a dispersion hardening process has been developed that involves dispersing some fine insoluble phase, usually refractory oxides, throughout the base metal matrix. This method is considered in detail in Chapter 14. The effect of precipitation hardening of an aluminum—4.5% copper alloy is shown in the following table:

Treatment

Yield Strength

Tensile Strength

30 Age hardened, 3

20 Overaged, 4